246 ME. J. A. DOL^GLAS ON GEOLOGICAL SECTI0:N'S [vol. Ixxvii, 



11. Geological Sections ilirougli the Andes of Peeu and 

 Bolivia: III — From the Port of Callao to the Eiter 

 Perene. By James Archibald Douglas, M.A., B.Sc, 

 F.a.S. (Read Ai^ril 20th, 1921.) 



[Plates XV-XX.] 



Contents. 



Page 

 I. Introduction _. 246 



II. Geological Description of the Section from Callao to the 



Eiver Perene 249 



III. General Summary and Conclusions 276 



IV. Bibliography 281 



I. Introduction. 



This paper deals with the geological structure of the Andes of 

 Central Peru, as represented by a horizontal section drawn from 

 the island of San Lorenzo off the Pacific coast at Callao to the 

 Hiver Perene, a tributary of the Apurimac, one of the headwaters 

 of the Ucayali. It is a third contribution to a series of papers 

 written as the result of a two j^ears' expedition to South America 

 (1910-12) undertaken on behalf of the late Mr. W. E. Balston, 

 F.G.S. 



The district traversed is one of the best-known and most 

 populous in Peru, for through it passes the northern Transandine 

 route from Lima, the capital of the country, to the port of Iquitos 

 on the Amazon, whence steamers sail direct to Europe. 



The first half of the section follows the line of the central 

 railway of Peru. This is the well-known Oroya line, famous not 

 on\Y as the highest railway in the world, but also as a wonderful 

 feat of engineering, rising as it does to an altitude of over 

 15,000 feet in a com-se of 100 miles. The line ascends the steep- 

 sided valley of the Pimae by means of a series of sinuous curves 

 and zigzag ' switchbacks,' eut for the greater part in the face 

 of the solid roc]^. After passing through no fewer than Q5 tunnels 

 it reaches the summit of the watershed between the Atlantic 

 and the Pacific river-systems at Ticlio, 1.5,QQ5 feet above sea-level, 

 and thence descending to 12,000 feet terminates at the junction 

 of Oroya. From Ticlio a branch-line leads to the important 

 mining centre of Morococha, situated in a valley remarkable for 

 the perfection of its glacial topography. At Oroya it makes 

 connexion with lines from the celebrated copper-mines of Cerro de 

 Pasco and from the agi'icultural districts of Jauja and Huancayo 

 farther south. In addition, it furnishes an outlet for the co:ffee 

 growing region of the interior. 



Leaving the railway at Oroya, the section is continued eastwards 



